CYCLOTRIMERIZATION OF STERICALLY HINDERED ALKYNES IN ULTRAHIGH-VACUUM- TPD OF TERT-BUTYLACETYLENE ON THE REDUCED TIO2(001) SURFACE

Cyclization of tert-butylacetylene on reduced TiO2(001) surfaces yield s one of the highest carbon-number products observed for thermally dri ven association reactions in ultrahigh vacuum. The C-18-cyclotrimer tr i-tert-butylbenzene was the principal product of tert-butylacetylene c onversion. The chemistry of tert-butylacetylene was essentially the sa me as that of alkynes containing less bulky substituents: open chain d imeric as well as hydrogenated monomeric species were observed in addi tion to the cyclotrimer. Both the dimer and trimer products have been attributed to the formation of metallacyclopentadiene intermediates at surface Ti2+ cations. The total conversion to oligomeric products for tert-butylacetylene was the same as that for methylacetylene, but the more hindered alkyne exhibited a lower trimer/dimer ratio. This compa rison indicates that the tert-butyl group does not affect the populati on of the intermediate metallacyclopentadiene species on the surface b ut does hinder the addition of the third alkyne necessary to produce t he cyclotrimer.